Can't get stopcock 100% closed to allow a tap repair

I have a plumber coming tomorrow to change a cartridge on a badly dripping tap on my bathroom mixer unit. There is no separate isolating valve for this tap so I'll have to close the stopcock. I tried closing it today but it won't go absolutely 100% closed and there is still a very slight flow from the tap. Will the cartridge still be able to be changed under those circumstances?

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